The present invention relates to a shift control apparatus of a belt type continuously variable transmission (CVT) installed in a vehicle.
A belt-type CVT is generally constructed by a primary pulley of receiving an engine revolution, a secondary pulley connected to driving wheels, a V-belt wound on these pulleys. A transmission ratio (CVT ratio) of the CVT is continuously varied by varying effective pulley diameters for the V-belt.
FIG. 8 shows a generally known shift schedule of a continuously variable transmission (CVT) and shows a relationship between a revolution speed of the primary pulley with respect to a vehicle speed Vsp, which relationship is a target of a throttle opening TVO. Herein, a primary-pulley target revolution speed Nprit corresponding to the combination of the throttle opening TVO and the vehicle speed Vsp takes a value within an area defined by a lowest-speed-side shift line derived from the mechanical limit of the CVT and a highest-speed-side shift line. The CVT is controlled so that the actual revolution speed Npri of the primary pulley is brought closer to the target revolution speed Nprit. As a result, a target transmission ratio ipt is represented and limited by a ratio between a primary-pulley target revolution speed Nprit and a secondary-pulley revolution speed Nsec (ipt=Nprit/Nsec). Therefore, the shift control of the CVT is executed to bring an actual transmission ratio ip closer to the target transmission ratio ipt.
In case that a vehicle starts on a low-friction coefficient (μ) upslope such as a snow-covered upslope, there is a possibility of inducing a wheel spin just after a start operation at moment t1 as shown in FIGS. 9A, 9B and 9C. Under such a wheel spin state, a conventional shift control executes operations that the target transmission ratio ipt is quickly changed to a high-speed side ratio in response to the quick increase of the driving-wheel speed Vw, and the actual transmission ratio ip is upshifted according to the quickly-changed target transmission ratio ipt. On the other hand, when a driver senses the wheel spin at moment t2 in FIGS. 9A, 9B and 9C, the driver stops a depression of an accelerator pedal so that the accelerator pedal is put in a released state so called a pedal release. By this driver's operation, a throttle opening TVO is fully closed. Therefore the driving-wheel speed Vw is quickly decreased and reaches 0 together with a vehicle body speed Vb, and at last the vehicle stops at moment t3.
In such a quick vehicle stopped state, the target transmission ratio ipt is quickly set at a low-speed side ratio according to the quick decrease of the driving-wheel speed Vw, and the transmission ratio ip is downshifted according to the quickly changed target transmission ratio ipt. However, the vehicle speed Vb is not so high, and therefore the vehicle stops within a short time period. Consequently, the vehicle stops before the transmission ratio is sufficiently returned to the low-speed side ratio, and is fixed at the high-speed side ratio at moment t3.
Under such a state that the transmission ratio ip is fixed at the high-speed side ratio, even if a restart operation of depressing the accelerator pedal is executed at moment t4 after the vehicle stop, the vehicle may not be able to restart due to the shortage of the driving force caused by the transmission ratio set at the high-speed side ratio.
Against the wheel spin on a low-μ road, there is a countermeasure of manually or automatically selecting a snow mode. By selecting the snow mode, a shift operation of setting the target input revolution speed Nprit within a low-speed side area represented by hatching in FIG. 8 is prohibited, that is, a shift operation of setting the target transmission ratio ipt within a low-speed side ratio area is prohibited even when the vehicle just starts. However, the operation executed by selecting the snow mode can not solve the above-discussed problem after the driving wheels have been put in a wheel spin state.
Japanese Published Patent Application No. H11-94071 has proposed a control method of ensuring a vehicle starting capability so as not to occur a shortage of the driving force during a vehicle start, by limiting the transmission ratio from changing to the high-speed side as compared with a set transmission ratio when wheel spin is detected after the vehicle start operation is executed. In this control method, it is determined that wheel spin is generated when the driving-wheel acceleration becomes greater than or equal to the set transmission ratio, and the set transmission ratio is set at a limit transmission ratio by which the vehicle can start on any travelable road such as on a low-friction-coefficient upslope. Further, the set transmission ratio is gradually varied to the highest-speed side transmission ratio so as to gradually cancel the limitation of the transmission ratio.